On Looking: Eleven Walks With Eyes Part 4

Sidewalks have sidewalk-crack ants. A graduate student who recently cataloged the ants in the medians of the long avenues in New York City found cornfield ants, thief ants, pavement ants (a warmongering, pavement-loving species), and a Chinese needle ant, a stinging ant that is not from around here. He also found that the ant diversity on the Upper West Side of the city was greater than on the Upper East Side-presumably a function of that great environmental condition, "number of trash cans."

Even fallen twigs can reveal insect sign. Surely you have twigs on the ground in your area? "Neatly severed twigs," as one of Eiseman's book sections is headed, can reveal the presence of a beetle girdler, a pruner, or a borer. While galls might happen to cause a twig to weaken and break, there are various beetles who intentionally weaken a twig: the girdler, after laying her eggs toward the tip, moves toward the tree's trunk and plows a tidy path around the circ.u.mference of the twig. When a wind comes along, the twig snaps off cooperatively and the growing larvae in the twig feed on the aged or dying tissue. In other beetle species, it is the larvae themselves who burrow, when they are old enough to begin burrowing, into the twig and chew their way to the surface, creating characteristic sign in the process. All you have to do is look at the end of the fallen twig to see the spiral formed by a hickory spiral borer, or the expertly cut burrow around the perimeter, caused by a different wasp.

The discovery of the day was not the downy woodp.e.c.k.e.r cruising up and down the hackberry in the corner of a vacant lot-leaving sign in the form of beak marks, itself sign of some tasty beetle under the bark. It was not the adorable pupa of a ladybug, sitting calmly square in the middle of a catalpa leaf, its head tucked under its body and its abdomen folded protectively.

It was the sign on wood by a most unlikely creature.

We had just found some slug slime on a birch leaf. "There's a slug among us," Eiseman said. I did not think of slugs as critters that might want to be on trees. But Eiseman described to me how some slugs eat the film of algae on the bark of a tree, and in sc.r.a.ping their teeth against the bark leave a series of kisses with jagged lips. The resultant mark shows up clearly on light-colored backgrounds, like a birch tree-or on a white propane tank, or an abandoned car covered with weather and detritus.

"It's a feathery pattern, this back-and-forth S-shaped pattern," he was saying, before, "oh, here we go."

On the broad trunk of the tree was a sinuous pattern of spikey footsteps, a series of stamps of a sharpened fern frond. Slug teeth marks.

Eiseman looked entirely satisfied. "I had always suspected it was slugs who were doing it"-leaving this kind of track-"but I couldn't figure out how, because I didn't realize slugs had teeth." It took seeing a slug in action to confirm his suspicion.

In truth, slugs do not really have teeth; they have radula, a finely toothed kind of tongue that only mollusks have. It allows them to graze, rasping their body against a surface to sop up whatever they are gliding over.

Sign of slug. It was pretty, delicate-even more so for being the unlikely result of a gelatinous, lumbering creature. We gazed admiringly at its path tattooed on the tree. I fumbled through my bag for a camera and snapped a photo of it, surely one of the only extant images of slug sign outside of Eiseman's and other slug enthusiasts' collections.

While we idled down a broad, newly laid sidewalk, hardly a sidewalk ant in its cracks, I began thinking about Eiseman's brain. What was it that allowed him to notice these mystery objects, where most of us see just plain leaves and twig and wall? How was it that lace bugs "jump out at" him while they left me staring blankly at a tree?

The difference between how Eiseman sees and how I see is traceable to a concept popularized in the early twentieth century by Luuk Tinbergen, brother of the n.o.bel Prizewinning animal behavior researcher Niko, and a noted bird-watcher in his own right. Tinbergen noticed that songbirds did not prey on just any insect that had recently hatched in the vicinity; instead, they tended to prefer one kind of bug-say, a particular species of beetle-at a time. As the numbers of young beetles rose through a season, the birds gorged on these beetlettes, ignoring any other available young insects nearby. Tinbergen suggested that, once the birds found a food they liked, they began to look just for that food, ignoring all others. He called this a search image: a mental image of a beetle-with its characteristic beetly shape, size, and colors-with which the bird scans her environment.

The concept of a search image has now been widely studied in the animal world and is used to help explain the efficiency many predators have in finding their prey, despite the best efforts of the prey to be unfindable. In the lab, blue jays trained to look for camouflaged moths initially have trouble seeing them-they blend in so well with the speckled bark on which they alight. But after a number of attempts, the birds get preposterously good at finding even the most well-concealed moths. Dogs, skunks, and spiders have been found to have olfactory search images: they are more concerned with the smell of their food than its shape, and can find, say, that dry dog food (dogs and skunks) or that particularly yummy mosquito (spider) among a riot of smells in the environment, by searching out its characteristic smell.

Search images are not just used or useful for finding prey or avoiding capture; they are the way we find our car keys, spot our friends in a crowd, and even find patterns that we had never seen before. The neurologist Oliver Sacks writes about a splendid, human example of this phenomenon from his own experience. At a time when Tourette's syndrome was not widely recognized, Sacks saw his first Tourette's patient, exhibiting the tics that define the syndrome. The following day, he says, "I saw three [ticcing] people on the streets of New York and another two the next day. And I thought, 'If my eyes are not deceiving me, this must be a thousand times commoner than it's supposed to be . . . why haven't I noticed this before?'" He had acquired a Tourette-tic search image.

Everyone needs a mechanism to select what, out of all the things in the world, they should both look for and at, and what they should ignore. Having a search image in mind is what makes finding your friend among the crowds of people disembarking trains at Grand Central Terminal possible at all: it is the visual form of the expectation that allows you to find meaning in chaos. At the same time, if you are searching for your friend who you last saw twenty years ago in high school, she may no longer look quite like your search image representation of her. Jakob von Uexkll, the German biologist, wrote about this with his own search for a pitcher of water, which he expected to find at his table at lunch. Though he was a.s.sured that the pitcher was in its usual place, he could not see it right in front of his face-for the clay pitcher he had expected had been replaced by a gla.s.s pitcher. The "clay pitcher search image" obliterated the perceptual image of the gla.s.s pitcher. Von Uexkll recognized this as the same mechanism that led animals to mistake harmless objects as fearsome. He described a jackdaw (a kind of crow) flying above bathers at the beach, fooled into attacking an innocent person carrying his bathing suit over his arm: the jackdaw had a search image for a jackdaw-in-a-cat's-mouth, and the wet, drooping trunks mimicked it. The jackdaw unreflectingly set to attack the feline killer of his brethren. Presumably our German biologist emerged with only minor peck marks.

Eiseman has an insect sign search image. He has got galls b.u.mpily imprinted on his mind; bug footprints etched in his brain. And in his eyes: neuroscientists who look at "visual search" find not only expected areas of the brain involved-a layer of the visual cortex called V4, the frontal eye fields (in the frontal cortex), areas in the brainstem and other areas involved in eye movement-but also the retinal ganglion cells in the eye itself. Our visual system has what researchers call inh.o.m.ogeneous processing; this is a fancy, and slightly unflattering, way of saying that even when we want to, we cannot see everything at once. We see best right in front of us, in the center of vision made crystal clear by the abundance of photoreceptor cells in the center of our eyes, the foveal area of the retina. The periphery of our vision? Not so much. Our eyes simply are not designed to focus on what is to our sides: that is why we have nicely swiveling heads (presumably evolution was not concerned with what was behind our heads). Once our eyes are open, we automatically begin scanning the environment, flitting our gaze to and fro in short saccades-quick, automatic hops of our gaze back and forth to move our two degrees of central vision across the fifty or so degrees of our future path. We gaze-hop to scan a scene and we gaze-hop simply to stay looking at the object in front of our noses. You cannot stop saccading (except with anesthesia to the eye), nor would you want to: if you looked steadily ahead without saccading to and fro, the image you were looking at would seem to disappear. After constant stimulation, our sensory receptors get tired and stop firing. The result is that we become inured to constant sensory input: we stop noticing the foul odor in a room (but it is still there for a while, as evinced by the expression of others entering the room after you) or the heat of the steam room air on our skin (though it is still the same hot temperature). Saccades are the eyes' way to avoid having the lion, mouth agape, disappear from our vision while we stand in front of him, frozen in fear.

Saccading and searching are normal behaviors, done every visual moment of every eyes-open day. Search images are for the masters. That is not to say that the masters always know how they use these search images to solve the puzzles before them. Interviews with expert radiologists, satellite image a.n.a.lysts, and fishermen show that though they recognize their own expertise, they often cannot tell you exactly how they identify the hidden malignancy, detect a target from a bird's-eye view, or spot the school of sardines as it surfaces. Me, I felt one step closer to being a gall search master myself-without knowing how.

As our walk wound down, I was reluctant to let Eiseman go. I slowed my pace to get him to talk about the insect sign on a limp plant by the side of a dusty road. It turned out to be a winding trail of one of two known grapeleaf moth miners. Following it, we found sweet purple grapes. We gobbled a handful. A melancholy thought occurred to me. Most people are not going to have an invertebrate tracker on their walks with them, I worried out loud. Eiseman reflected for a moment, and then quoted one of his tracking teachers, Susan Morse: "Half of tracking is knowing where to look, and the other half is looking." If you understand even the most superficial elements of the life histories of different animals-such as what kinds of things they are attracted to-once you start looking, you are going to find them everywhere. If you want to find otters, find a peninsula in a big body of water where they might scent-mark and loiter; if you're looking for rats, try the alleyways behind restaurants, where overflowing bags of trash are left, kitchen floor mats are shaken out, and busboys eat hasty meals on break. Hence the ease at finding gulls around Dumpsters, racc.o.o.ns sheltered in the holes in a stone wall, and, we discovered, parasitized flies on the bottoms of leaves in Springfield.

A small bit of knowledge goes a long way when thinking about "where to look": "Every kind of mammal has a particular landscape feature or microfeature that it keys in on, where it does its marking behaviors," Eiseman explained. "With insects it's just the same, just smaller: microhabitats." You need only know those habitats. Once you have an eye for those features, whether alley, leaf, or wall, as long as your eyes are open, you are likely to see the animal or the traces it has left. Eiseman described coming across the sign of a palmetto tortoise beetle while on his cross-country insect voyage: "I had seen a picture in a book of this beautiful structure that a palmetto tortoise beetle makes-it's a beetle larva that only feeds on palmetto leaves. It extrudes this long strand of brown segmented excrement [which might only be beautiful to palmetto beetles and Eiseman], this twisted ma.s.s of segmented straw. I didn't know what size it would be. My brother and I were walking along a trail in Georgia and I-I wasn't specifically looking for it-I just spotted one out of the corner of my eye; it was that big"-he held his fingertips almost touching-"about eight feet away. . . . Somehow it jumped out even though it seems physically impossible to see it."

After our walk, I had a bit of the sensibility of a spider: find a corner and build a web to catch insects bopping around. I had a search image for a sinewy trail on bark or leaf that would indicate a leafminer or a barkminer. This intuition is not always desired, I will admit: after our first encounter, I found a dozen motionless, parasitized flies on the undersides of leaves. That is an image I will be happy to remember how to ignore. But once you have an eye for these things, even when you're not looking for them, they just jump out at you. Everything is a sign of something.

"It did seem like the more we just stopped in one spot . . ." I began.

". . . we just started seeing more things," Eiseman, my tour guide to the bugs, finished the thought.

1 As a member of an invasive species myself, the tone of this descriptor seems unduly harsh.

"It matters not where or how far you travel-

the further commonly the worse- but how much alive you are."

(Henry David Th.o.r.eau).

The Animals Among Us.

"A spill of spaghetti, cooked and sauced, formed a sunburst at my feet, attended to by a cl.u.s.ter of pigeons."

It was December twenty-first, the winter solstice. The business of being a pedestrian in the city had changed: any mosey that crept into people's summer gait had been replaced by the determined fast stride of the winter walker. It was cold out, and I hunched my shoulders in a futile attempt to warm my ears and bully the chill away. But I was walking slowly enough to scan the tree branches and windowsills and fence tips for squirrels. For December twenty-first is the first day of the mating season for the eastern gray squirrel, apparently. Perhaps I would see some courts.h.i.+p, a typically pell-mell affair in which many males race after a single female squirrel-up tree trunks, along delicate branches, leaping across wide creva.s.ses. It sounded like the rodent version of a great chase film, without the small European cars and tourist-crowded plazas. At the very least, the squirrels should be out and about on this monumental date, engaged in something other than the nonstop nut-collecting and -consuming that has been their occupation for the last months. I knew this because John Hadidian told me so.

He is Senior Scientist in the Wildlife division of the Humane Society and had flown up from Was.h.i.+ngton, D.C. to meet me. Even to those of us familiar with seeing pigeons and squirrels on our daily walks, looking for "wildlife" in a city may sound oxy-moronic. But New York City, the most densely peopled city in the country, is, like all other urban environments, host to a huge population of nonhuman animals. The bears, wolves, and mountain lions that roamed the land that became our city are no longer here (we think). But squirrels, racc.o.o.ns, bats, deer, fox, coyote, possums, and scores of bird species are. Hadidian and I aimed to see if they were, what is more, on my very block.

Like most animal behavior researchers, Hadidian began his scientific career studying a relatively exotic animal-in his case, the crested black macaque, an Old World monkey you have likely never seen outside of a zoo. Just when he was looking for a job, though, the National Park Service was hiring, trying to get a handle on the racc.o.o.n population, which, like the rabies the animals were carrying, was skyrocketing. So Hadidian became, all of a sudden, a rare breed of animal researcher: one who studies the most common, least exotic animals. He began tracking and observing the urban racc.o.o.n more than twenty-five years ago, and he never went back to primates.

Happily for me, though, he readily agreed to walk around a city block with this human primate. Also like most animal behavior researchers, Hadidian is full of a kind of animal trivia that comes from long hours observing, reading, and discussing animal habits and quirks. Studying the macaques, Hadidian cataloged their yawning behavior and found that the males not only yawned much more often than the females, but they nearly always yawned in response to thunder. This kind of gem can only come from many, many hours of watching and yawn-counting.

A compact man with a gentle smile, Hadidian stepped out of a cab dressed in two, maybe three layers of sweats.h.i.+rts, as befits someone whose profession involves standing outside for hours at a time. We were not a minute into our walk before his squirrel trivia came up.

"One thing I'd want to comment on-aside of the fact that there's a squirrel's nest here," he began, gesturing with his head up in the bare branches of a gingko tree. A dense cl.u.s.ter of dried leaves smudged the silhouette of the tree against the sky. I took the bait.

"How do you know it's a squirrel's nest?"

"Just by the way it's built-it's messy-and how it's positioned in the tree. It's a leaves nest, a drey, which is not at all cuplike . . ."

. . . and he dove into a veritable natural history of the squirrel. It was in this way that I learned that this very day was the first day of our arboreal squirrel's breeding season, one of two it engages in per year.

Just today! The specificity of the claim was astonis.h.i.+ng. Though as city residents we come to expect to see animals outside . . . somewhere . . . whenever we leave our homes, it is another thing to think of them living through their own datebooks, their own calendars of things-to-do and when to do them. But just as there is a time of year when we uninstall the air conditioner and send it to the bas.e.m.e.nt, or flock to a Florida beach to warm our chilled northern skin, the animals around us are habitual as well.

The mating-season news got my hopes quite up for squirrel sightings. But the wind was whistling through the nest. It looked abandoned: perhaps last season's nest site, housing its handful of hairless squirrelettes for two or three months. No squirrels remained. And Hadidian had moved on, too, back to the topic that the squirrel's nest had interrupted.

"I was going to say that the primary distinction in the city is between night and day."

For urban wildlife, that is. Though we think the city is fundamentally the same place in the nighttime as in the daylight, it is not. It is not just darker, cooler, and quieter; it is teeming with animals. As plentiful as the pigeons, the sparrows, the chipmunks, or the squirrels may seem to be on city streets, what you see outside by day is a fraction of what you would see along the same route at night.

The reason for this is simple: us. "Humans are predictable, in terms of their behavior," Hadidian went on. "We create pulses of traffic: we're going into town, and then we're going home. And long about one thirty, two thirty [in the morning], things really quiet down." At night, we retreat from the streets, pull into our sh.e.l.ls, tuck ourselves cozily in bed. And that is just when the animals' city day begins.

It is a logical move for animals. If they are to live around us, they must adapt to our galumphing presence. We are rather noisy, for instance: in an urban environment the ambient din is regularly 50 to 70 decibels, with spikes to 100, a point at which a sound can be both physically painful and destructive to ears. Where Cooper's hawks, a North American bird of prey, have settled around humans, as in New York City, Was.h.i.+ngton, D.C., and San Diego, the birds have resorted to vocalizing more frequently, ensuring that at least some of their calls are heard by other Cooper's hawks. The great t.i.t, a small bird common in the Netherlands, and the song sparrow in the United States both sing at higher frequencies in cities, as most of human-produced sound is at relatively low frequencies. Urban scrub-jays breed earlier (giving them more time to recover from unexpected losses) and forage more efficiently than their country cousins. Some animals have changed themselves with lightning speed. The most famous case comes from the UK at the time when cities became industrialized. With factories belching black smoke, a layer of soot settled on every surface, man-made or natural. Among the population of the small peppered moth-Biston betularia-which counted on its coloration to be camouflaged against tree bark, a rapid transformation occurred. Where once the peppered coloration was dominant, suddenly a rare black varietal made up the majority of the population. These black moths were better camouflaged against the soot-coated trees. Birds couldn't spot them, and the little black moths flourished.

There are good reasons for animals to live around us humans. We provide plentiful food resources. We create shelters that easily accommodate small, discreet animal homes. But humans are also, for the most part, predatory, disruptive, and destructive. In our ordinary lives, we eat animals, kill them with our cars, and disturb their trash-can meals and garden-dirt baths. Thus, some animals have become more crepuscular-active at dawn or dusk-to avoid us. And more have become entirely nocturnal, even if the same species in rural areas is out and about in the day.

The first ones out at night are the racc.o.o.ns.

You might think of racc.o.o.ns as suburban animals, but if you live in a temperate city with a park, chances are you live quite close to a few hundred of these cat-sized mammals. Racc.o.o.ns are cla.s.sic urban adapters. They are generalists: they are so unpicky that they will live anywhere and eat nearly anything. In the city, the racc.o.o.n avails himself of the convenient supply of all things edible in our city trash bags, left insecurely tied on the streets twice a week. While a dog or rat might open a bag and scatter its contents, the racc.o.o.n tears a small hole, reaches in, feeling around for what he wants, and pulls it out. Hadidian noted that the animals he observed ate almost anything-except raw onion.

They are also alarmingly intelligent: "They're what I call the North American primate," Hadidian said. Watched closely, the behavior of Procyon lotor is indeed oddly familiar.1 They sit upright, grasping food in their front paws-which, with five fingers, look like and function remarkably like human hands. You might catch a racc.o.o.n dexterously fingering an item; or gripping, exploring, and fiddling with it. Their reflexes are better than ours-a racc.o.o.n can s.n.a.t.c.h a fly out of the air-but give one a Rubik's cube and he will, like so many humans, turn it methodically and not solve it. Like young persons, racc.o.o.ns love to play-with their own tail tips or with each other; with a corncob rolled between their front paws; by drumming on the floor with their fists. One racc.o.o.n was spotted attempting to tie a bit of straw around his nose. Their facial mask-the familiar gla.s.ses of black hair around their eyes-adds to the feeling that they have distinct personalities. Should you be stealthy enough to come upon a sleeping racc.o.o.n, you will likely find it in one of two positions: either flat on his back, with his front paws over his eyes; or on his stomach, his head tucked between his front limbs as if frozen in the beginning of a somersault. I observed strikingly racc.o.o.nish sleep poses in my own husband and toddler last night.

As a culture, we are decidedly ambivalent about the racc.o.o.n. "Criminals!" some cry, surely accusing these masked bandits of more than they are able of being. But racc.o.o.n researchers speak of their subjects' "knavery," greed, and curiosity. We seem caught between identifying the racc.o.o.n as wild animal or as an urban pest. On the one hand, it is cute, costumed and catlike; on the other, it is a disease-carrying scourge. Even a century ago, racc.o.o.ns were fairly beloved in America. They were popular as pets and were known as mischievous, inquisitive, and quick studies. President Coolidge, sent a racc.o.o.n for his Thanksgiving table, promptly decreed her his pet, named her Rebecca, and took her with him on long walks and on whistle-stop train tours. The sight of a crowd of children gathered around the First Lady clutching Rebecca to her chest on the White House lawn for the annual Easter egg roll, an image captured in the newspapers of the time, seems about as unlikely today as one featuring the current president cozying up to a brown rat.

I asked Hadidian to name the most surprising thing he noticed about the social life of his racc.o.o.ns, the ones he had tracked and radio-collared for decades. "Culturally, these racc.o.o.ns were a little like lions," he replied. Though thought of as solitary animals, urban racc.o.o.ns live in groups. The pressures of city life lead to this crowding. They have many different den sites-in sewers, bas.e.m.e.nts, hollow centers of stone walls, and holes of trees. One animal that he tracked even traveled well outside of his home range each fall, to build a den near a beautiful persimmon tree, a huge wildlife lure: "Every animal you can think of is drawn to the persimmon tree," Hadidian said. They notice the smell of the tree's fruit, surely, but even outside of their olfactory range they remember where this special treat is and migrate toward it.

Hadidian stood by the curb, gazing down, it appeared, at the sewer grate. I reluctantly joined him.

"There. Someone is living in there."

Although it was hardly put there to deter wildlife, the city sewer system would seem to be an inauspicious place for an animal. But the sewer, with its curbside drains and pipes running along every street and connecting to every building, is home to innumerable racc.o.o.ns, rats, and possibly possums (rats willing). Plenty of the racc.o.o.ns Hadidian tracked made homes in the Was.h.i.+ngton sewer, which sweeps away the excreta of some of the country's most powerful people.

"A couple of the animals we followed actually gave birth and raised young there," he said.

"Coming out . . . where?"

"Oh, anywhere." He pointed at a small gap between a step leading into a neighborhood bodega and the sidewalk. Bits of debris clogged the opening, which did not look in the least hospitable to living and raising young racc.o.o.ns.

"A racc.o.o.n could easily fit in there, even at a full run."

I tried to imagine the racc.o.o.n's tail disappearing under the step. It is nigh impossible to picture a racc.o.o.n wanting to, or actually darting into that forlorn s.p.a.ce. This is what makes the urban animal so elusive. He is actually attempting to elude us, and our imaginations do not seem to account for animals (aside from pets) in cities. Even our sense of scale is distorted when considering urban wildlife corridors and pa.s.sageways. Remembering, perhaps, a childhood inability to scale a fence or s.h.i.+mmy through a gate, we find it incredible that urban animals are not thwarted by the seemingly impenetrable stone walls and chain-linked barbed-wire fencing we present to them. But the descriptions of nearly all urban animals include an impressive dimension: the size hole the animal can squeeze into, through, or out of. Racc.o.o.ns, even as adults, can fit in a four-inch s.p.a.ce between grates, flattening themselves and taking advantage of their broad, short skulls. Squirrels fit through a hole the size of a quarter; mice, through dime-sized holes. Look around you on your next walk. See any holes at all? Gaps between stair and building? Between sidewalk and curb? An animal goes there (after you have pa.s.sed).

Racc.o.o.ns are not only widespread but also long lived for urban animals. Hadidian followed the first animal he radio-collared for thirteen years. For the time it takes a child born in the city to learn to sit up, to stand, to walk; to say simple words, complex words, to talk, to talk back; to text, to do algebra, to play a tune on the piano-during all that time, he may have been quietly accompanied by a racc.o.o.n living a parallel life (with less algebra) outside, within a block of his home.

As I looked around me, seeing no racc.o.o.ns but all the places they might be-in twelve hours' time-I spied a charming bra.s.s lion's head at eye level along a granite wall. One cannot help but notice that it is never a handsome racc.o.o.n's head emboldened in bra.s.s as a knocker. But racc.o.o.ns have these leonine ways, and are at least as charismatic (and less predatory on humans, to say the least). If the city were overtaken with lions, instead of racc.o.o.ns, I wonder if we would feel the same way about them.

We had been walking for blocks, and apart from evidence of squirrels past and racc.o.o.n accommodations, the only animals I had seen were ones dressed in puffy black parkas and snow boots. I asked Hadidian if we would have to come out at night in order to see the real wildlife of the city.

"Well, we've heard some house sparrows, saw one starling, saw a big flock of pigeons. . . ."

What? This was news to me. "You should tell me about this!"

I smiled, abashed. Even when looking for animals, I had missed them. All three birds are ubiquitous. Our starlings are famous-not for their great iridescent coloration, or even the peek of bright blue on the beaks of the males, but for their being an ecological disaster. "They were brought over [to the U.S.] by a fellow named Eugene Schieffelin who wanted to bring to North America every bird that was mentioned in Shakespeare," Hadidian added. Starlings make their appearance in Henry IV. "Nay, / I'll have a starling shall be taught to speak / Nothing but 'Mortimer'," says Hotspur-evidence that Shakespeare knew of the starlings' ability to mimic speech. Clever birds-so clever, unfortunately, that they wreaked havoc on their new environment. Before they proliferated, according to Hadidian, "most of the birds of the eastern U.S.-maybe thirty-four hundred species" traveled through countryside that became Manhattan.

How had I missed these birds? It had to do with how I was looking. Part of what restricts us seeing things is that we have an expectation about what we will see, and we are actually perceptually restricted by that expectation. In a sense, expectation is the lost cousin of attention: both serve to reduce what we need to process of the world "out there." Attention is the more charismatic member, packaged and sold more effectively, but expectation is also a crucial part of what we see. Together they allow us to be functional, reducing the sensory chaos of the world into unbothersome and understandable units.2 Though we think of expectation as a cognitive experience, the process actually operates on a single-cell level, and even within the sensory organ. Consider it from, say, the eye's vantage. A stimulus. .h.i.tting the cells of our retina could be one of many things. The eye is agnostic about what it is: a toy car held on the palm of your hand looks the same to the eye as a real car seen from a distance out the window of a building. But we never confuse the two. It is context (am I holding this car or looking out the window?) that allows us to resolve the ambiguous pattern into "matchbox" or "bona fide." That's expectation's doing.

Attention and expectation also work together to oblige our missing things right in front of our noses. There is a term for this: inattentional blindness. It is the missing of the literal elephant in the room, despite the overturned armchairs, dinner-plate footprints, and piles of dung. Psychologists have cleverly demonstrated our propensity to miss a rather obvious element of a visual scene when attending to another by asking subjects to watch a specially designed short video. In this video, two teams, dressed in white or black s.h.i.+rts, toss around a basketball. The task is to count the number of tosses made by one of the teams. That is the expectation: the viewers expect there'll be basketball-tossing! They gear up to see it. Afterward, the subjects are asked for their final tally.3 Of course, this is not the actual question of interest to the researchers. That question is this: Did you, attentive subject, notice anything else? Anything unusual? Anything else . . . at all?

Nearly half of all subjects did not. In this case, the elephant in the room is an actual gorilla-well, a person in a gorilla costume-who waltzes right between the players, pounds his chest, and saunters off-screen. Paying attention to the basketball players, we miss a rather salient (and furry) figure among them.

Expectation also allows us to miss bits of the ordinary world, not just the gorillas in our midst. Indeed, it nearly prevents us from seeing lots of things happening around us. In an economic move, rather than try to process everything we see, our eyes stop spending time taking in information from those parts of the environment that hold steady. Staring at your computer screen, or the book in your hand, your eyes quickly stop processing all of the details of the monitor or the corners of the pages in depth. If something changes, sure, the eye darts to it and neurons fire away. If nothing changes, those neurons can go quiet. In this way, expectation suppresses activity in the sensory system. This is usually not only unproblematic but helpful: by limiting our perception to what is likely, we see what we need to more quickly and reliably. A carriage horse wears blinders to restrict his visual world physically, since carriage drivers long ago learned of the inefficacy of simply asking the horse nicely to Please focus on what's straight ahead, never minding that cute filly to the side. People who perform highly unusual and difficult memory feats-memorizing thousands of digits of pi, for instance-may wear goggles or headphones to block distracting sensory information from interfering with the parade of numbers in their heads.

Blinders and goggles are ways of physically restricting what you can, and expect, to see in the world; the brain has its own internal mechanism. By thinking about what you are looking for, or antic.i.p.ating what you might be looking at, your brain grows biased to see it: biologically, the neuronal processes are primed to spot objects that fit your expectation. The object does not need to be simple: think about a face, and you enhance activity in special brain cells that process faces-leading to seeing the face faster and more clearly than you otherwise would have. This, combined with the "search images" that helped Charley Eiseman find the insect tracks invisible to me, enables us to transform the world: expectation magically sorts the world into things-we-are-looking-for and things-we-are-not.

This phenomenon of expectation was discovered through a simple test called the "cued-target detection task." It is a very boring version of a war-games video game. The player is asked to press a b.u.t.ton when he notices a target pop up: some simple thing like a flas.h.i.+ng light or a triangle on the screen. On different rounds, there will be different cues to where the target might be: sometimes an honest cue, which appears right where the target will be; sometimes a misleading cue, which appears clear across the screen from the target.

Unsurprisingly, the honest cue helped people respond about twenty-five milliseconds faster to the target than the misleading cue.4 But in either case-whether the cue is misleading or not-people simply saw the target faster with a cue, because they knew to expect it. The effect is mediated by various neurotransmitters, which start rallying neurons to fire earlier than would otherwise happen.

And we do not expect to see animals in the city. So we for the most part do not. If we notice things faster simply by expecting them, then looking at the world as if it holds "cues" for us just might work. I endeavored to test this theory at once, with urban wildlife in mind. My gaze fell on a well-graffitied wall to my left. It encased a schoolyard, full of young, s.h.i.+n-guarded soccer players, prancing around on Astroturf. The wall seemed hardly worthy of careful examination, but with my heightened keenness for signs of animals, I felt there was something there. A small, uneven piece of wire mesh was jammed between and over the bricks. It covered, barely, a small gap where the bricks converged. Hadidian followed my gaze and offered his a.s.sessment.

"I think that is intended to prevent someone's access."

A pause, and then, as though answering his own question, he continued: "Yeah, it's probably a rodent barrier. Not a great one. Kind of stuffed in there."

It was not a rat, but a past rat! Very likely, the presence of a surfeit of rats prompted this makes.h.i.+ft protection. The handiwork was, indeed, not impressive. We paused to examine it. The mesh was spiderwebbed across the wall-another piece was right below it, another was a yard to the right. Though there was no sign of the animals it aimed to prevent access to, if anything, these protuberances from the wall seemed likely to draw a rat's attention to the site. Hmm, the pa.s.sing rat might wonder. Where to enter this fine-looking den site of a wall?. . . Aha! At the great mesh gates!

I remembered seeing Hadidian sigh, two blocks before, as we pa.s.sed a trash can on the sidewalk. Not an unusual trash can-in fact, a rather tidy one, as resident trash cans go here-its lid was just ajar over the yawning can. An "insecure" trash can, he had called it. Almost guaranteed to be visited by the local racc.o.o.ns that night.

Sure enough, on further examination we saw recently chewed bits of paper peeking out through the mesh on the wall. This was certainly the work of rats, whose ever-growing teeth leave them yearning to gnaw, chew, and pulp whatever they can get their mouths around.

It appeared that we had stumbled across one of the ways to find wildlife-or the traces of wildlife-in the city: look for the traps, barriers, or other munitions set up to deter them or ward them off. As Hadidian and I continued down the street, our attention was drawn upward by a rustling sound. On a flagpole, someone had tied strips of plastic, which whirled spastically in the breeze. Nearby windowsills and protruding ledges were lined with unfriendly spikes to deter animals' loitering-"loafing," as it is called by animal experts. While intended to discourage loafers, the plastic strips seem to attract them: as we watched, three pigeons, Columbia livia, landed nearby and made themselves comfortable. Pigeons can often be found at just those places that feature pigeon-scaring tactics. A plastic owl peered out placidly from a fire escape. While conceptually this seems like it would dissuade loafing pigeons-they are potentially prey to the predator owl-pigeons easily learn that an immobile, silent, odorless owl is not to be feared. And, indeed, the owl may be a sign of the location of a nice, stable ledge, with access to the crumb leavings of a nearby resident human. As Hadidian put it, the owl effigies "provide pigeons with something nice to sit on."

Similarly, the positioning of rodent boxes-large, enclosed traps baited with poison and, with their rat-sized openings, intended to lure rats-is itself a bit of urban ethology. The superintendents or residents who put them out presumably place them where they have seen rats in the past, so the location of these boxes forms an unlikely map of the paths of the city rat. On one block, I noticed that the view behind an apartment building extended all the way to the next street south: somewhat rare in New York, a city without alleys. I pointed this out to Hadidian, who said, simply, "Corridor. That's a great animal corridor." That is, it was a potential access route for animals, who would like to travel but prefer to avoid the traffic of the sidewalks and streets. At bas.e.m.e.nt level we saw a string of rodent boxes lined up along the back walls of the buildings open to this ersatz alleyway. This was a rat superhighway.

The boxes spoke not just to the rats' route, but also to their idiosyncrasies. All the boxes were lined up against the building walls because rats are thigmotaxic: a splendid word to describe an animal who likes to walk along walls, touching something as it goes. Thigmotaxic, or thigmophilic-touch-loving-animals scurry along at the edges of the s.p.a.ces we make. They feel most comfortable keeping in contact with something as they travel. You might spot the smudges of their body oil left as they hug the walls while moving. Mice are thigmotaxic; so are c.o.c.kroaches. It is the same phenomenon, worn slightly differently, that causes caterpillars to move in ma.s.sive processions, one touching (or clambering over) the caterpillar to his fore, who himself touches the caterpillar in front of him, and so on and so on.

This fact, in turn, tells us something about the biology of the animal. Thigmotaxic animals are probably ones who use the presence of a wall to orient themselves or navigate. Their sensory systems are often acute, but proximity to a wall limits the territory they must scan for possible dangers. And it makes use of one of their abilities: for rats, it is their ability to pick up low-frequency sounds-essentially, vibrations-with their vibrissae, or whiskers. These modest, twitching face hairs are actually the rat's best means of discovering what is out there, how far it is, how fast it is moving, and how high it is. More sensitive than human fingertips, whiskers can gauge, say, the size of an opening a rat is ducking into. If their whiskers are damaged, the animals are truly disabled. Researchers have experimentally cut lab rats' whiskers and found these animals to be not just slower to learn how to navigate through mazes but also likely to drown outright if placed in water.

We humans try not to touch things on the street, in public, but whiskers want to touch-to get information about the world through contact. So if you are looking for a whiskered animal, you need to envision the places that allow for lots of touching. Squeezed-into s.p.a.ces. Too-small holes. Leafy areas. Chewed-out corridors. Pipes, gutters. Curbs.

And then we saw one. A Norwegian rat, Rattus norvegicus, peeking over the curb edge. Surprisingly large, balanced on his hind legs, his front raised thoughtfully, he had spotted us first. His nose was in the air-investigating us-and his tail was down, a long lean curl of stability.

I stood still. I neither wanted him to come closer nor to run away. Though they live near us, rats are wary of our aversion to them, and are quite happy to avoid actual contact with people. The rat bobbed his head, looking a bit maniacally twitchy. Pigeons head-bob, too, though with more soldierly rigor, in rhythm with their footsteps. Both animals do it to see better: with each bob, they get different vantages on what is in front of them, and they can use that information to estimate depth. Standing still, the rat's vision is very blurred, compared to ours, although as befits their habitat they can see better at night than we can. By bobbing, he was getting a gauge on how far away I was in order to estimate when he needed to begin to flee.

We were both silent, though I wondered if he was squeaking over my head, as it were. Rats' primary communication with other rats is along ultrasonic, high-frequency wavelengths above our hearing range: there is a mother-retrieval call by young pups; a "long call," which accompanies pain or social defeat, and, oddly, e.j.a.c.u.l.a.t.i.o.n; and a high chirp that has been interpreted as a laugh, emitted during tickling by humans (in the lab) and during play. They also chatter their teeth, an inadvertent communication of stress. They can certainly hear us. Some rats can distinguish different languages. Resident rats in the bas.e.m.e.nt of an apartment building with a building staff speaking a different language than its residents can tell from their speech which group is possibly threatening to their den, and which will ignore them.

This rat must have decided I was unworrisome, for he soon relaxed into a sit and began wiping his face and ears with his fore-paws. Rats spend up to half their waking hours preening. The ritual keeps their coats covered with their musk, and keeps the parasites at bay. They begin grooming at their heads and proceed downward, so you can get a sense how far along the rat is in his self-shower by seeing what part of his body he has gotten to cleaning. My rat got no farther: a bicycle rattled by on the street and he dove under a parked car. He was gone-back into the s.p.a.ces rats inhabit where they can be invisible to us.

An animal behaviorist to the core, Hadidian took an even-handed approach to the animal.

"From a strictly natural history perspective, they're one of the most poorly understood animals out there."

"What is there to understand about them?"

"How big are their home ranges? How much socializing do they do? What's their group structure? . . . We really don't know what the behavioral repertoire of the rat is."